Document MAME ROM and front end folders

[milnak]

Front end folders (from what I've figured out):

• snaps in rpi2jamma/snaps
• videos (from progetto hyperspin videos) in rpi2jamma/video

MAME 0.106 folders (also from what I've figured out):

• MAME 0.106 roms in rpi2jamma/roms/arcade
• Can't figure out where samples go?!? I tried under rpi2jamma/roms/arcade/samples but they don't seem to be picked up?
• Can't figure out if artwork supported (e.g. for asteroids)? If so, what folder?

/root/.advance/advmame.rc reads:

• dir_rom /mnt/sda/rpi2jamma/roms/advmame
• dir_sample /mnt/sda/rpi2jamma/roms/advmame/sample
• dir_snap /mnt/sda/rpi2jamma/snaps_advmame/snaps

but none of those paths didn't work for me.